When conventional aqueous dispersion compositions for coating are coated to the surface of an object and then dried, the coating film shrinks due to evaporation of the solvent or water and cracking occurs. That is, development of mud crack often occurs.
As suggested in WO97/40112, in order to prevent mud crack, a specific polyether urethane resin is used as an auxiliary for forming film. However, although mud crack is decreased, because thermal decomposition of the urethane groups is insufficient by the baking temperature and time suitable for a fluororesin coating film, undecomposed residue remains in the coating film, causing problems such as coloring of the obtained melted coating film.
JP-A-50-8828, JP-A-51-60243 and JP-A-52-13531 suggest using depolymerizable acrylic resin particles as an auxiliary for forming film. In the aqueous dispersion compositions for coating described therein, mud crack is prevented by using acrylic resin having a low glass transition temperature to prevent mud crack and furthermore, by using butyl carbitol together as a water-soluble high boiling point solvent for plasticizing the acrylic resin and dissolving the acrylic resin by the butyl carbitol concentrated in the drying step. When the glass transition temperature of the acrylic resin is largely decreased, decomposability becomes poor and therefore using together with an oxidant is described.
The present inventors have considered prevention of mud crack by a solution from a physical viewpoint and not a chemical formulation such as adding resin as a film forming component or adding butyl carbitol. The present inventors have found that development of mud crack can be prevented by occupying a specific proportion of void, based on the filling conditions and void ratio of the resin particles of the resin aqueous dispersion composition after drying, with a nonionic surfactant having specific thermal properties, without depending on the kind and density of resin or chemical properties of a nonionic surfactant. Thus the present invention was accomplished.